The invention relates to an electrochemical half-cell which at least comprises a membrane, an electrode, which may generate gas, as anode or cathode, optionally an outlet for the gas, and a supporting structure which joins the electrode, which may generate gas, to the half-cell rear wall. The supporting structure divides the interior of the half-cell into vertically arranged channels, the electrolyte flowing upwards in the electrode channels facing the electrode and flowing downwards in the channels facing away from the electrode, and the electrode channels and the channels facing away from the electrode being connected to one another at their top ends and at their bottom ends.
Incomplete or incorrectly performed gas separation in the upper region of prior art electrolytic cells will lead to inadequate wetting of the membrane at this location and an increase in the electrical resistance of the membrane. This results in an increase in the integral cell voltage and additionally carries the risk of local membrane damage due to so-called xe2x80x9cblisteringxe2x80x9d. The damage to the membrane can be so significant as to allow electrode gas to pass through and, possibly, explosive gas mixtures to form. Moreover, erroneous gas separation may give rise to pressure surge pulses in the electrolyte compartment, resulting in membrane movements wvith a risk of premature ageing due to mechanical damage.
A further problem is that of operating the electrolytic cell employing as homogeneous a vertical and horizontal temperature and concentration distribution (salt concentration or pH of the electrolyte) as possible in the region of the electrolyte compartment upstream of the membrane surface, likewise in order to avoid premature membrane ageing. This is generally desirable for the operation of all gas-generating electrolysers, but especially for the use of gas diffusion electrodes in which the heat dissipation (removal of lost heat) must take place predominantly or entirely via the electrolyte circulation on the other, gas-generating side, depending on whether a finite electrolyte gap (finite gap) or a resting gas diffusion electrode is employed beyond the membrane. This may involve a reduction in the temperature of the incoming fresh electrolyte for the gas-generating side, which must not lead to local overcooling here.
In the past there have been a few proposals for mitigating these problems, albeit only for the classic hydrogen-generating NaCl electrolysis. For example, the European Offenlegungsschrift (European Published Specification) EP 0579910 A1 discloses a system to induce an internal natural circulation, especially in order to render acidification of brine for the NaCl electrolysis more effective and to reduce excessive foaming in the upper region of the electrolytic cell.
The European Offenlegungsschrift (European Published Specification) EP 0599363 A1 discusses various methods of dealing with gas bubbles caused by the process, without mentioning the decisive elements which enable complete separation of gas and electrolyte at the same time as entirely pulsation-free and even joint outflow of the separated phases from the cell and which enable equalization of temperature and concentration right into the corners of the cell.
The solution of these problems of the known electrolytic half-cell arrangements is achieved by a half-cell according, to the precharacterizing clause with the characterizing features of the independent claim.
The present invention relates to an electrochemical half-cell at least comprising a membrane, an electrode, which may generate gas, as anode or cathode, and a supporting structure which joins the electrode, which may generate gas, to the half-cell rear wall as well as an inlet for the electrolyte and an outlet for the electrolyte and optionally for the gas, characterized in that the supporting structure divides the interior of the half-cell into vertically arranged channels, the electrolyte flowing upwards in the electrode channels facing the electrode and flowing downwards in the channels facing away from the electrode, and in that the electrode channels and the channels facing away from the electrode are connected to one another at their top ends and at their bottom ends.
In particular, the channels carrying a downward flow and the electrode channels are arranged alternately next to one another or else behind one another.
In this arrangement, the channels carrying a downward flow and the electrode channels can have a trapezoidal cross section.
Preferably, the channels carrying a downward flow and the electrode channels are formed by a folded metal sheet, an electrically conductive one, as a supporting structure.
In a particularly advantageous embodiment of the half-cell, the electrode channels have a cross-sectional constriction at their top ends.
A vertically aligned, parallel supporting structure in a specific arrangement separates the channels which are open towards the electrode and in which the lighter electrolyte-gas mixture is rising, from channels which are open towards the rear wall and in which the degassed, heavier electrolyte flows downwards again. An essential feature to improve the gas separation is a constriction located herein at the top of the electrolyte channels, which is produced by an aerofoil wing-like flow deflector profile which is curved towards the electrode. The two-phase flow is accelerated in the constriction between electrode and profile, is expanded above the rearward curved top edge of the profile and is degassed on the rear of the profile while phase separation takes place. On its rear, the profile exposes orifices into the downcomer channels, so that the heavier electrolyte, heavier because it has been degassed, flows downwards and at the half-cell bottom, via communication orifices, flows as the gas-absorbing fraction, together with electrolyte freshly fed in, into the channels which are open towards the electrode, and thus effects the internal natural circulation of the electrolyte.
Preferably, the cross-sectional area of the electrode channels in the narrowest region of the constriction in proportion to the cross-sectional area of the electrode channels below the constriction is from 1 to 2.5 to 1 to 4.5.
The constriction of the electrode channels can be formed, for example, by an angled guide structure.
In particular, the constriction of the electrode channels has a region of constant cross section, the height of this region being at most 1:100 in proportion to the height of the active membrane surface.
Fabrication of the half-cell is possible in a particularly simple manner if the guide structure and the supporting structure form one piece.
Equally advantageous is a design of the half-cell in which the supporting structure is in the form of one piece over the entire height of the electrode channels and the channels carrying a downward flow.
Advantageous for gas separation from the electrolyte is a design in which the electrode channels above the constriction have an expansion of their cross sections.
The excess electrolyte leaving the cell can be discharged, downstream of the flow deflector profile, either laterally at the top or downwards via a vertical pipe.
Particularly advantageous, therefore, is a half-cell which has an outlet for the degassed electrolyte and any gas formed during the electrolysis, in particular a vertical pipe with a through-hole in the cell bottom, or an outlet disposed on a side wall of the cell, said outlet being disposed just above the top ends of the electrode channels.
As experimental experience shows, it is most especially advantageous for the overall structurexe2x80x94apart from the communication orifices right at the bottom and the communicating gap having a width of a few mm above the profile right at the topxe2x80x94to consist of a functional unit in order to fulfil the following functions:
Separation of the gas bubbles from the electrolyte via the so-called xe2x80x9cbubble jetxe2x80x9d at the top in order to enable discharge of electrolyte and product gas separately or alternatively jointly as separated phases, but above all without any pressure pulses
Equalization of the vertical temperature profile by means of a vigorous natural circulation over the full height in order to optimize the membrane function
Equalization of the vertical concentration profile via the same mechanism in order to optimize the membrane function
Equalization of the vertical pH profile, e.g. in the case of systematic acidification of the brine in NaCl electrolysis in order to improve the chlorine yield and quality. Local over-acidification of the brine would be damaging to the membrane
In addition to the hydraulic function, the supporting structure assumes the function of mechanically retaining the electrode and in addition the function of low-resistance connection of the electrode to the cell rear wall.
In a preferred variation, the supporting structure together with the electrode channels and the downflow channels fills the interior of the half-cell to at least 90%.
Preferably, the supporting structure is electrically conductive and is connected electroconductively to the electrode and to in particular to the rear wall of the half-cell.
The electrode is then preferably connected electroconductively to the supporting structure of the half-cell and is mounted on the supporting structure.
For the purpose of regulating the temperature of the electrolyte, upstream of the inlet of the electrolyte there is preferably a heat exchanger via which fresh electrolyte and optionally degassed electrolyte recirculated from the outlet are introduced into the half-cell, thus forming a temperature-controlling electrolyte circulation if required. The pressure-surge-free and complete separation of the gas bubbles, in conjunction with the equalization of temperature profile, concentration profile and pH profile gains particular significance when gas diffusion electrodes are used in one of the half-cells, be it on the anode or cathode side, in the case of a gas-generating process on the other side of the membrane. In these cases, dissipation of the ohmic lost heat must take place largely or entirely via the electrolyte from the gas-generating side of the electrolyser, depending on the type of operation of the gas diffusion electrode.
The electrolyte processed in the anode compartment is e.g. an aqueous sodium chloride solution or a hydrochloric acid solution, the anode gas produced in the process being chlorine. The counterelectrode is an oxygen-consuming cathode.
If, e.g. with NaCl electrolysis, an oxygen-consuming cathode having a narrow catholyte gap is operated on the cathode side, as described in EP 0717130 B1 and follow-up patents, cathode-side heat dissipation can take place only via plug flow without turbulence, shifting the heat balance more towards the anode side, if one wishes to avoid employing excessive cathode-side heating intervals, which are known not to benefit the membrane. Here it is therefore necessary either to operate with a cooled electrolyte in a single-feed arrangement or alternatively, if appropriate, with a likewise cooled anolyte circulation, in order to keep temperature distributions inside a cell at the optimal level.
If e.g. an NaCl or alternatively HCl electrolysis is performed with resting oxygen-consuming cathode, cathode-side heat dissipation is marginal; the heat must be dissipated virtually entirely via the anolyte. This generally requires an external anolyte circulation with cooling.
In all these cases, particular significance is attached to internal equalization of temperature, concentration and possibly pH, since the amount of electrolyte fed into the cell increases relative to the internal circulation, so that the latter must be particularly intensive in order to avoid anything being askew, even just locally. This particularly applies to a, quite desirable, hefty acidification of the brine in the case of NaCl electrolysis, said acidification generally having to be carried out in line with the lowest local pH.
If the half-cell having a finite catholyte gap is operated upstream of an oxygen-consuming cathode, some of the lost heat can be dissipated on the cathode side via the flow through said catholyte gap and external cooling, while the predominant fraction of the lost heat is dissipated with the anolyte stream.
If, on the other hand, the half-cell is operated with an oxygen-consuming cathode resting on the membrane (zero gap), the entire lost heat is dissipated via the anolyte stream.
Further advantages of the half-cell according to the invention are therefore the vertical equalization of the temperature of the electrolyte and the vertical equalization of the electrolyte concentration.
The half-cell according to the invention can be used generally in all gas-generating electrolyses. It gains particularly significance in electrolyses in which electrolyte and gas can be separated from one another only with some difficulty.
The invention is explained below in more detail, by way of example, with reference to the figures without the invention thereby being limited in any specific point.